The present application claims priority of German Patent Application 101 18 420.4, filed on Apr. 12, 2001, the disclosure of which is hereby expressly also made the object of the present application.
1. Description
The invention relates to a drive unit for a machine, more especially an injection molding machine for processing plastics materials and other plasticizable substances.
2. State of the Art
DE 199 20 626 A1 makes known this type of drive unit, where a plurality of linear motors are stacked to form one drive unit. The linear motors are cylindrical and magnets are disposed on the lateral surfaces of the rotor as well as stator windings being disposed on the lateral surfaces of the stator. A plurality of lateral surfaces with the same action are interconnected and are operated in common.
Tests have shown that by stacking together a plurality of linear motors which are connected to form one drive unit, although simultaneously the power can be increased and there is a compact design, the drive unit in operation exhibits a sinusoidal force characteristic. The sinusoidal force characteristic is produced by the basic principle of magnetic elements of the linear motor disposed in rows in the rotor and includes a wave length which corresponds approximately to the length of the magnetic element. To eliminate this effect, DE 100 20 338 A1 consequently already proposes that where there is a plurality of linear motors operated in parallel, the angle of the electric motors is offset in a stepped manner. Nevertheless production tolerances and wear and tear can result in oscillations in the force characteristic.
DE 197 29 976 A1 has already established that where there is linear displacement, the coupling between the magnetic poles, on the one hand, and the conductors of the current-carrying coils, on the other hand, is not constant in dependence on the geometric position of the two parts relative one to the other, i.e. for example when the two parts are displaced. This means the tangential force generated in dependence of the location or respectively of the position of the displaced part is no longer constant . Test results show oscillations in the force characteristic from up to 40%.
To solve this problem, it has already been proposed in DE 197 29 976 A1 that the terminal edges of the magnetic elements be sloped or respectively that a plurality of part magnets, which are rectangular in shape, be offset one relative to the other along the surface of the rotor.
U.S. Pat. No. 5,973,459 A specifies a required force characteristic for a linear motor in dependence on the path position of the linear motors. In this case it is assumed that the formula of the magnetic field characteristic is known. A force determining means is not available.
A linear motor for use in an injection molding machine is also proposed in EP 0 280 743 A1, however the application in that case purely tackles the problem in FIGS. 4 to 7 of how the displacement of the linear motor is generated when the various phases are connected.
Proceeding from this state of the art, an exemplary embodiment of the invention is a drive unit for a machine which shows a steady force characteristic when a linear motor is used.
In this solution attempts are made to balance out the differences in the feeding force which are location-dependent using automatic control technology. The force characteristic is regulated over the path of the linear motor by the power of the individual linear motors being controlled via a power unit. To this end, at least at assembly, preferably also when the linear motor is operating, the force characteristic is determined with a force determining means. The results obtained in this manner are used in the operating state to control or respectively regulate the output of the power units, such that the desired force characteristic is produced. Consequently, there is an xe2x80x9cintelligentxe2x80x9d system, which can smooth the force characteristic independent of limiting conditions and production tolerances.
This automatic control solution can be superposed by a mechanical solution, where the magnetic elements and/or coil elements of the linear motors, which are connected in parallel relative to one to another, that is different linear motors which together form the drive unit, are offset relative one to the other. This means that the force characteristics of the different linear motors can be superposed in such a manner that as steady a force characteristic as is extensively possible is produced.
A further smoothing of the force characteristic can be achieved, where applicable, also independently of each other, i.e. for example with different stepped stages, the magnets are offset in a stepped manner along the circumference of the lateral surfaces of the linear motors.